A travelling-wave optical amplifier of a 1.5-micrometer band with an Er (erbium) added optical fibre has a high efficiency, no polarization dependency, and an excellent feature of match with a transmission system. Its practical application to excitation of laser diodes (hereafter LD) of a 0.98-micrometer and 1.48-micrometer bands has vigorously been studied. As a result of experimental studies of signal-to-optical gain and noise characteristics, it is already known that higher-efficiency and lower-noise characteristics are achieved by the excitation in the 0.98-micrometer band than the excitation in the 1.48-micrometer band. Development is, however, currently in progress as regards an optical amplifier comprising an optical device (as, for example, an optical isolator) suitable in the 1.48-micrometer band.
As a 0.98-micrometer optical isolator, optical isolators are in practical use at present with use of a terbium-gallium-garnet (TGG) single crystal. They are too bulky as compared with an LD and are not studied for future points of view. Bulk yttrium-iron-garnet (YIG) and Bi-substituted garnet are highly absorbing at the 0.98-micrometer band and are not practical because of an insertion loss of about 5 dB. It has been confirmed that Cd.sub.l-x Mn.sub.x Te is a material having a great Verdet constant and is confirmed to exhibit a practical performance as a material for optical isolators of visible wavelengths of 0.85 to 0.63 micrometer when Mn is substituted for a part of Cd of CdTe of a ZnS crystal structure (Onodera and Oikawa, the 15-th Scientific Lecture Proceedings of the Applied Magnetism Institute of Japan, 30aB-7, page 179 (1991)).
The Verdet constant is, however, too small at 0.98 micrometer and has made it difficult to use in practice. This has been a main reason of a delay in development of an optical isolator for 0.98-micrometer band excitation of LD-moduled optical amplifier despite its intrinsically higher efficiency and lower noise characteristics than 1.48-micrometer band excitation.
It is therefore an object of this invention to provide a magnetooptical element suitable for use in a wavelength range between 0.8 micrometer and 1.1 micrometers (above all, 0.98 micrometer).
It is another object of this invention to provide an optical isolator in which use is made as a Faraday rotator of the magnetooptical element of the type described.